Radio receiver



Feb. 15, 1944. J. E. MAYNARD mno REmvER Filed June 29', 1942 K 2 shets-sne'et 1 Feb. 15, 1944. J, E MAYNARD 2,341,937

RADIO RECEIVER Filed June 29, 1942 2 Sheets-Sheet 2 CARRIER WAVE SOURCE inventori JQhn E'. Magnard.,

Patented Feb. 15, 1944 RADIO RECEIVER `Folin E. Maynard, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York nppiieanon .time 29, 194,2, sei-iai No. 448,881

(ci. 25o-zo) 9 Claims.

My invention relates to carrier wave receivers and more particularly to such receivers adapted for reception'of both frequency modulated and amplitude modulated signals in which a stage utilized as a limiter when receiving a frequency modulated signal is used-as a source of voltage the variations of which may be utilized for automatic volume control lpurposes when an amplitude modulated signal is received. l* has for one of its objects to provide a limiter circuit for such a receiver which is capable of acting on weak signals. y

In radio receivers that are designed for the reception of either frequency or amplitude modulated carrier Waves, it is desirable that the switching of the receiver circuit from one type of reception to the other be accompanied with a minimum of disturbance of the tuning of the preceding circuits of the receiver. another object of my invention to provide a radio receiver in which all switching from one type of reception to the other is effected in the unidirectional potential connections without any disturbance of radio frequency or intermediate frequency tuning. Y

A further object of myinvention is to provide a diode type of limiter fora radio receiver which sharply limits the intensities of frequencyv modulated carrier waves without loading the following circuits of the receiver and which may be easily adapted, by means of switching in the direct current circuits of the diode whenv the receiver is employed for th'e reception of amplitude modulated carrier waves, to provide a continuous potential for automatic volume control purposes.

The features of my invention which believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together With further objects and advantages thereof,- may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig. 1 represents a receiver for frequency modulated carrier Waves embodying my invention; Fig. 2 represents a receiver for both frequency modulated and amplitude modulated carrier waves embodying my invention; and Fig. 3 represents a modification of the receiver of Fig. 2. As shown in Fig. 1, high frequency carrier Waves are supplied to th'e amplifier" l, either directly from a receiving antenna-,or through a channel such as a radio frequency amplier. In the usual form of super-heterodyne receiving apparatus they are carrier waves having the inter- He'nce it is v mediate frequency of the receiver and may be supplied from a preceding intermediate frequency amplifier.

The high frequency carrier waves are subjected to amplitude limitation in the limiter circuit 2. They are then demodulated in a detector and discriminator 3 and the demodulated signals, which areordinarily audio frequency signals, are supplied to a low frequency signal amplifier il. From the output of the amplifier d the signals may go to further stages of amplification or may be coupled'directly, as sh'own, to a sound reproducer 5.

The amplier l comprises an electron dischargevdev-ice having a control electrode ll coupled `to a source of frequency modulated Waves, a cathode i2, and an anode I3. The anode i3 is coupled by means of capacitor lli to the limiter 2. The limiter consists of a pair of diodes l5v and I6 connected with polarity reversed with respect to each other so that one conducts during positive peaks and the other during negative peaks of intermediate frequency voltage. The inductance I1 provides a direct current return path for the diodes so that they function as rectiers. A bypass capacitor I8 is connected between inductance l'l and the anode of diode i5, While a bypass capacitor I9 is connectedbetween inductance Il and the cathode of diode l. The value of inductance Il is so adjusted that, when combined With the straycapacities of the diode circuits, these circuits resonate at the frequency of the carrier wave. As a result, when the diodes are non-conducting, th'e limiter 2 presents a high impedance to the anode circuit of amplifier i. The bias batteries 20 and 2| prevent the diodes from operating at small intermediate frequency voltages. The capacitor I4 serves to block the anode potential of amplifier I from the limiter diode. Since there is no direct current load circuit for these diodes other than their own internal impedance, when the intermediate frequency voltage rises above the bias voltages 2li and 2l, these diodes present practically a short circuit to the intermediate frequency amplier l.

The anode I3 is likewise coupled by means of impedance 22 to th'e primary 23 of the discriminator transformer. This impedance 22, shown as comprising an inductance 24, and a resistance 25, serves both to allow the limiter to cut off sharply the peaks of voltage Waves and to prevent the limiter, from short-circuiting the discriminator transformer.

The discriminator circuitv 3 is of well known form. Its function is to demodulate the frequency modulated waves which are coupled thereto from the anode I3. The circuit comprises a transformer having, in addition to the' primary winding 23, a secondary Winding 26, both of these windings being tuned to a desired fixed intermediate frequency. The primary winding isconnected between ground and the mid-point `of the secondary Winding 25 through a condenser 2l. Unidirectional operating potential for the anode I3 is supplied through the resister 29 from the positive terminal of a source of potential shown as a battery te. The opposite terminals of the secondary winding 26 are connected to the respective anodes ofthe diodes 3l and 32, the cathodes of these diodesv being connected together for alternating current through condenser 33 and for direct current through resistances 34 and 35i. is grounded andthe mid-point between resistances 35i `and 35 is connected to the mid-point on secondary winding 2S through a choke coil 36.

In the operation of the discriminator circuit, when the tuning control of the radio receiver is adjusted for accurate resonance with the received carrier waves, the intermediate frequency has a desired value to which the primary and secondary Windings 23 and 26 are each tuned. The voltage across the secondary winding of the transformer, in accordance with Weil known theory, is displaced in phase with the voltage across the primary by 90. With the connection shown, the voltage of the primary is connected in series with one-half the Voltage on the secondary across the discharge device 3i and the condenser 33 and it is connected in series with the other half of the voltage on the secondary across discharge device 32. Because of the quadrature relation between the primary and secondary voltages, the voltage on one-half of the secondary leads the voltage on ktheprimary by 90, whereas that on the other half of the secondary lags behind the-voltage on the primary by 90. Thus, the voltage applied to the two diodes 3| and 32, when the intermediate frequency is at its desired value, is equal and accordingly equal values of unidirectional current iiow through each ofthe diodes and hence through resistances '34 and 35. It will be observed that these resistances are poled oppositely, that is, the voltages across the two are opposite in polarity` in the vcircuit between the cathode of discharge device 3l and ground, with the result that the cathode of discharge device 3i is at ground potential when the intermediate frequency is at the-desired value.

The quadrature relation between the primary and secondary voltages of the discriminator transformer exist, however, only when the oscillations supplied thereto have the desired intermediate frequency. If this frequency changes in either direction the phase of the secondary voltage varies from its 90 relation with the primary voltage in one direction or the other, dependent on whether the frequency increases or decreases. For example, if the frequency increases, the phase shift may be in such a direction that the voltage on the upper half of the secondary winding approaches the aiding relation with the primary voltage, whereas that on the lower half of the secondary winding approaches the opposing relation with the primary voltage. Thus, the voltage applied to diode 3l' increases and that appliedto diode. 32'. decreases with the result'that the unidirectional potential on re- The cathode of the diode 3E jvolume control resistor 38. The output of signal amplifier i may be increased through succeeding stagesL of amplification, if desired, or may be, as shown, coupled directly into an audio transformer 3e, the secondary of the audio transformer being connected to reproducing means such as a loud speaker lill.

In the operation of my limiter circuit, the pair ci diodes i5 and I6 being connected in reverse polarity, the one conducts on positive peaks and the other on negative peaks of intermediate frequency voltage. The inductance l'l providesa direct current return path to these diodes so that they function as rectiiiers. At the same time, this inductance is turned with the circuit and stray capacities of the limiter to a resonance condition so that the limiter circuit presents a high impedance to the anode circuit of amplier I while the diodes areV non-conducting, the bias batteries Z and 2l preventing the diodes from operating at small intermediate frequency voltages. Since there is no direct current load circuit for these diodes, other than their own internal impedance, when the intermediate frequency voltage rises above the bias voltages, the diodes present practically a short circuit to the intermediate frequency ampliiier l. The impedance 22 performs the important function of -both preventing the limiter from short-circuiting the discriminator transformer and allowing the limiter to ychop peaks of intermediate frequency voltages abruptly. The limiter ycircuit being' tuned broadly, it has a small fly-wheel effect, that is. a small circulating current; hence, the limiter responds sharply to small .changes in voltage. Since the discriminator transformer, on the other hand, has a much greater inertia or fly-wheel effect, the limiter circuit and the discriminator transformer would have detrimental inter-acting effects for the presence of the greater y-wheel effect of the discriminator transformer would be detrimental to limiting. At the same time, the

loading represented by the limiter would reducev the` ability of the discriminator to build up circulating current, with a resultant lowering of its quality and sharpness of tuning. These effects are greatly reducedl by the insertion 0f the coupling impedance 22 having an impedancevalue as high as or higher than that of the primary of the discriminator transformer.

One of the advantages of this limiter is that it limits at very small voltages in the anode circuit of the iinal intermediate frequency amplifier l.

Such early limiting isan important factor inobtaining the advantages of frequency modulation insofar as the reduction of noise is concerned. Also, since the iinal intermediate frequency amplifier is of the pentode type, having a high plate impedance and operating in a'con# stant current manner into a-low impedance load, such as the discriminator transformer, itwill be seen that the introduction-of the impedance 22 causes very little reduction gain-and'makesl but a small change in the voltage produced across the primary of the discriminator'transformer. Also, since the amplifier I operates at normal unidirectional potential, its amplifying properties are not compromised byA the action of'fa limiter circuit. i

In the circuit shown in Fig. 2,'representing a receiver for both frequency modulated and amplitude modulated carrier Waves, the limiter 2 functions in the above-described manner.v The biasing voltages for the diodes l5 and I6 respectively are obtained from the potential drops across resistors 42 and 43 in the cathode circuit of the preceding amplifier I. By means of these resistors, the anodes ofthe two diodes are maintained negative with respect tothe cathodes, preventing current flow into these diodes for-a no signal condition in the receiver. For `the reception of amplitude modulated signalV waves the inter-connected switches44, 45,46, 41', and 48 are shifted from the contacts marked F to those marked A. With this shift the resistors 49 and .56 are inserted respectively in the circuits of diodes I5 and I6. These resistances have a large value so thatA when current iiows in the diode,

a suiiiciently high impedance ispresented to theV anode of amplier I that limiting does not occur. The circuit of diode I6 can now be traced through resistor 56, switch 45, resistor 5I, resistor 52 in the cathode circuit of amplifier 4,` switch 46 and inductance Il. It will also be seen that the anode of diode I6 is now connected through switch 41 and resistor 53 to the carrier .wave source. rThe potential vdeveloped across resistor 59 may be used by means of this circuit to control the gain of the preceding stages of `the receiver. The potential across resistor 52 serves to maintain the cathode of diode I6 positive with respect to the anode so that until the intensities of the signals across diode I6 are greater than a predetermined intensity, no current will flow through diode I6 to set up thepotential across resistor 55 for automatic volumeicontrol purposes. Also, by the shifting of switch 44 to the contact A, the cathode of diode I5 vis connected through resistor 54 to the source of 1potential30, thus establishing a positive bias between thecathode and anode of diode I5 to prevent conduction of current by the diode.A

It will be seen that the intermediate frequency amplier now operates in a normal manner, all

switching in the limiter circuit having been ef-l fected at unidirectional potential-,without disturbing the intermediate frequency tuning.- It will be seen also that the automatic volume control circuit precedes the audio signal circuit, thus providing automatic volume control selectivity broader than signal selectivity.

To obtain an amplitude modulated carrier wave voltage for detection purposes, a third winding 55 is provided on the discriminator transformer. for tuning the discriminator-primary circuit to the intermediate frequency while capacitor 58 is tuned with coil 55 to the intermediate frequency. Capacitor 57 couples the primary circuit to the circuit of winding 55.- The amplitudeA modulated carrier waves in the resonantcircuit of the coil 55 and capacitor 58 are impressed across the diode 69 for demodulation. The output current of the diode 60 establishes potential drops across resistors 6I and 62 which vary in accordance with the amplitude of the audio frequency currents flowing in diode 60, capacitor 59 providing a bypass across these resistors for radio Capacitors 56 and 51 are presentfrequency currents. Ey means o'f switch 48 thev audiovoltages across resistor 62 are supplied to the coupling capacitor 31 connected to the control electrode of signal amplifier 4. It will be seen from the above description 'that the switches 44-48 all operate in the direct current circuits of the diodes of the limiter 2 and the detector 69.

In the modication of the invention shown in Fig. 3, instead of being derived from a third winding of the discriminator transformer and a special detector, audio voltages are supplied to the signal amplier 4 from the potential variations across resistor 49 in the circuit of diode I5. In this circuit intermediate frequency currents flow from the lanode I3 of the amplifier I through the coupling condenser I4, a tuned circuit comprising thek inductance I1 and the diode I5 and bypass condenserY I8, through switch 46 in its right hand position, resistor 52 and bypass condenser 63 in the cathode circuit of signal ampli- Iier 4 and the bypass condenser 64 in the cathode of the intermediate frequency amplier I. Unidirectional currents flowing in the diode I5 through the resistor 49 and inductance Il to the anode of the diode provide audio frequency volt- I age variations across resistor 49 which are supplied by means of switches 44 and 48 in their right hand positions, and coupling condenser 31 'to the control electrode of signal ampliger 4. In all other respects the circuit of this modification is the` same -as that of the receiver illustrated in Fig. 2.

Thus, it will be seen thatmy invention provides a receiver which, lwhile allowing limiting on weak signals, permits the last intermediate frequency amplifier to operate under optimum conditions. This is provided by the use of the diode limiter and the decoupling impedance which permit sharp limiting without unduly loading the discriminator circuit. Moreover, by means of the switching arrangement shown, the limiter circuit is lconverted to an automatic volume control rectier for amplitude modulated Waves. Modification of the circuits to permit reception of amplitude modulated waves or frequency modulatedk Waves is eiTected by a simple switching arrangement in the direct current circuits of the vaudio `frequency stages of the receiver, thus avoiding any disturbance of the tuning of the radio frequency stages of the receiver.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modiiications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in a radio receiver, of a source of signal modulated carrier waves, a limiter, said limiter comprising a plurality oi' diodes arranged to transmit opposite peaks of said waves, a frequency discriminator, and impedance means connected between said limiter and said discriminator, said impedance means comprising an impedance connected serially betweensaid limiter and said discriminatorl and having a magnitude suiiicient to decouple said limiter from said discriminator to prevent the v,inertia of said discriminator from aiecting the limiting action of said diodes.

2. The combination, in a receiver for both frequency modulated and amplitude modulated carrier Waves," of means for detecting said Waves, a signal channel through Which said Waves are supplied to said detecting means, a diode connected in shunt to said channel, means for biasing said diode during reception of frequency modulated waves to be non-conductive to waves of less than a predetermined intensity and conductive to Waves of greater than said predetermined intensity whereby said diode limits said waves to said predetermined intensity, a resistance, and means to remove said bias and to connect said resistance in series With said diode during reception of amplitude modulated waves thereby to produce a unidirectionalv voltage on said resistance having an intensity varying with the intensity cf the received vcarrier Wave, and means responsive to said unidirectional voltage to control the transmission characteristics of said channel.

3. The combination, in a radio receiver for either frequency modulated or amplitude modulated carrier Waves, of an amplifier for such Waves, detecting means for such waves connected to the output of said amplifier, a unilateral conducting device connected across the output of said amplifier, means to bias said unilateral conducting device during reception of frequency modulated waves to limit said Waves to a desired value, means to remove said bias and to connect a resistance in series with said unilateral conducting device during reception of amplitude modulated waves thereby to produce a unidirectional voltage on said resistance, and means to vary the amplification of said amplifier in ac'- cordance With said unidirectional voltage.

4. The combination in a radio receiver for either frequency modulated or amplitude modulated carrier Waves, of an amplifier for such waves, detecting means for such waves connected to the output of said amplifier, a unilateral conducting device connected across the output of said amplifier, means to bias said unilateral conducting devicev during reception of frequency modulated waves to limit said waves to a desired value, means to remove said bias and to connect a resistance in series with said unilateral conducting device during reception of amplitude modulated Waves thereby to produce a unidirec-V tional voltage on said resistance, means to varyl the amplication of said amplifier in accordance.

with said unidirectional voltage, and means connected between said unilateral device and said detecting means to prevent said unilateral device from short-circuiting said detecting means during reception of said frequency modulated Waves,

5. -The combination, in a radio receiver, of a source of signal modulated carrier waves, a limiter, said limiter comprising a plurality of diodes arranged to transmit opposite peaks of said Waves, and a tuned circuit connectedto the output of said limiter through an impedance comprising a resistance and a reactance con,

nected in parallel, said impedance having a magjnitude of the order of the impedance of said tunedcircuit to prevent the inertia of said `tuned circuit from affecting the limiting action of said diodes. K

6.l The combinatiomin a receiver for both frequencyI modulated and 4amplitude modulated carrierwaves, Pof'means for detecting said Waves, a signal channel through which said Waves are supplied to said detecting means, a diode connected in shunt to said channel, means for biasing said diode during reception of frequency modulated Waves to be non-conductive to waves of` less than a predetermined intensity and conductive to waves of greater than said predeter-y minedfintensity whereby said diode limits said Waves to said predetermined intensity, a resistance, and means to remove said bias and to con-A nect said resistance in series With said diode during reception of amplitude modulated waves thereby to prevent said diode from limiting the amplitude of said amplitude modulated waves.

c 7. The combination, in a radio receiver for both frequency modulated and amplitude modulated carrier waves, ofan amplifier for such Waves,y detecting, means for such Waves connected to the output of said amplifier, a unilateral conducting ldevice connected across the output of said amplifier, means to bias said unilateral conductingdevice during reception of frequency modulated Waves to limit said Waves to a desired value, means to Aremove said bias and to connect a resistance in series with said unilateral conducting device during reception of amplitude modulated Waves thereby to prevent said device from limiting the amplitude of said amplitude modulated waves.

tov

8. The Acombination in a radio receiver for both frequency modulated and amplitude modulated carrier Waves, of an amplifier for such waves, a unilateral conducting device connected across .the -outputvof said amplifier, means to bias said device during periods when said receiver is receiving frequency modulated waves to limit said Waves to a desired value, a tuned circuit connected to the output of said amplifier through an impedance, said impedance being of sufcient magnitude to prevent the inertia of said,A tuned circuitfrom affecting the limiting action of said device, and means vto remove said bias and ,to connect a resistance in series with said device during periodswhen said receiver is receiving -amplitude-modulated waves to pre,-

vent said device from limiting the amplitude of.

said amplitude modulated waves.

, 9. The combination, in a receiver for carrier waves modulated in frequency in accord with audio frequency signal currents, of a converter and a channel through which said carried wave is transmitted to said converter, said converter being adaptedto` reproduce said audio frequency signal currents from said frequency modulated carrier Waves, means-connectedacross said channel having high impedance to carrier waves of loW intensity and lowV impedance to carrier Waves of high intensity thereby to limit carrier Waves suppliedto said converter to values not exceeding a predetermined level, and an impedance` in seriesrwith said channel between said last means and said converter of a value suf-` 

